As a photolithography stepper ages, a distortion of the photolithography stepper becomes non-linear, in other words, different visual fields shows different amounts of magnification, rotation, and offset X/Y. In daily monitoring, the largest visual field is usually selected. The visual field range of photolithography templates produced by chip design companies is very broad, which determines the field size differences between different products of wafer factories, resulting in an alignment difficulty during the processing of the wafer, and is likely to shift when being aligned.
In order to address the above defects, a conventional solution is to adjust the lens of the photolithography stepper, and improve the distortion. However, the improvement is quite limited, while the cost is expensive, and the distortion may still go worse over time. Alternatively, it is also a solution to perform compensation according to measured overlay values of different products. Nevertheless, products of wafer foundries renew fast, and have many varieties; such solution requires a lot of manpower, which is ineffective. Moreover, during a first processing of a new product, the compensation value is unable to be pre-estimated, sacrificing the alignment accuracy and circulation speed of the new product when reworking is needed for out of specification.
Referring to FIG. 1, FIG. 1 is an aligning and monitoring view of a conventional photolithography stepper. When performing aligning and monitoring according to such method, i.e., selecting a visual field 1 to perform monitoring, in which an alignment measurement pattern 11 is put at four corners of the visual field 1, and performing compensations such as rotation, magnification, and offset according to the overlay value measured by the alignment measurement pattern 11 at the four corners. Such method can perform accurate compensations to the product close to its visual field 1, but when the visual field of the product deflects the selected visual field of the monitor, misalignment is likely to occur. In addition, the conventional method is expensive, ineffective, and difficult to perform alignment control.